ASTM F1321-1992(2004) Standard Guide for Conducting a Stability Test (Lightweight Survey and Inclining Experiment) to Determine the Light Ship Displacement and Centers of Gravity o.pdf

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1、Designation: F 1321 92 (Reapproved 2004)An American National StandardStandard Guide forConducting a Stability Test (Lightweight Survey andInclining Experiment) to Determine the Light ShipDisplacement and Centers of Gravity of a Vessel1This standard is issued under the fixed designation F 1321; the n

2、umber immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.INTRO

3、DUCTIONThis guide provides the marine industry with a basic understanding of the various aspects of astability test. It contains procedures for conducting a stability test to ensure that valid results areobtained with maximum precision at a minimal cost to owners, shipyards, and the government. This

4、guide is not intended to instruct a person in the actual calculation of the light ship displacement andcenters of gravity, but rather to be a guide to the necessary procedures to be followed to gather accuratedata for use in the calculation of the light ship characteristics. A complete understanding

5、 of the correctprocedures used to perform a stability test is imperative to ensure that the test is conducted properlyand so that results can be examined for accuracy as the inclining experiment is conducted. It isrecommended that these procedures be used on all vessels and marine craft.1. Scope1.1

6、This guide covers the determination of a vessels lightship characteristics. The stability test can be considered to betwo separate tasks; the lightweight survey and the incliningexperiment. The stability test is required for most vessels upontheir completion and after major conversions. It is normal

7、lyconducted inshore in calm weather conditions and usuallyrequires the vessel be taken out of service to prepare for andconduct the stability test. The three light ship characteristicsdetermined from the stability test for conventional (symmetri-cal) ships are displacement (“displ”), longitudinal ce

8、nter ofgravity (“LCG”), and the vertical center of gravity (“KG”). Thetransverse center of gravity (“TCG”) may also be determinedfor mobile offshore drilling units (MODUs) and other vesselswhich are asymmetrical about the centerline or whose internalarrangement or outfitting is such that an inherent

9、 list maydevelop from off-center weight. Because of their nature, otherspecial considerations not specifically addressed in this guidemay be necessary for some MODUs.1.2 This standard does not purport to address the safetyconcerns, if any, associated with its use. It is the responsibilityof the user

10、 of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2. Terminology2.1 Definitions:2.1.1 inclining experimentinvolves moving a series ofknown weights, normally in the transverse direction, and thenmeasuring the re

11、sulting change in the equilibrium heel angle ofthe vessel. By using this information and applying basic navalarchitecture principles, the vessels vertical center of gravityKG is determined.2.1.2 light shipa vessel in the light ship condition (“Con-dition I”) is a vessel complete in all respects, but

12、 withoutconsumables, stores, cargo, crew and effects, and without anyliquids on board except that machinery fluids, such as lubri-cants and hydraulics, are at operating levels.2.1.3 lightweight surveythis task involves taking an auditof all items which must be added, deducted, or relocated on theves

13、sel at the time of the stability test so that the observedcondition of the vessel can be adjusted to the light shipcondition. The weight, longitudinal, transverse, and verticallocation of each item must be accurately determined andrecorded. Using this information, the static waterline of theship at

14、the time of the stability test as determined frommeasuring the freeboard or verified draft marks of the vessel,the vessels hydrostatic data, and the seawater density; the light1This guide is under the jurisdiction of ASTM Committee F25 on Ships andMarine Technology and is the direct responsibility o

15、f Subcommittee F25.01 onStructures.Current edition approved July 1, 2004. Published July 2004. Originally approvedin 1990. Last previous edition approved in 1992 as F 1321 92.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.ship displ

16、acement and longitudinal center of gravity can beobtained. The transverse center of gravity may also be calcu-lated, if necessary.3. Significance and Use3.1 From the light ship characteristics one is able to calcu-late the stability characteristics of the vessel for all conditionsof loading and ther

17、eby determine whether the vessel satisfiesthe applicable stability criteria. Accurate results from a stabilitytest may in some cases determine the future survival of thevessel and its crew, so the accuracy with which the test isconducted cannot be overemphasized. The condition of thevessel and the e

18、nvironment during the test is rarely ideal andconsequently, the stability test is infrequently conducted ex-actly as planned. If the vessel is not 100 % complete and theweather is not perfect, there ends up being water or shipyardtrash in a tank that was supposed to be clean and dry and soforth, the

19、n the person in charge must make immediate deci-sions as to the acceptability of variances from the plan. Acomplete understanding of the principles behind the stabilitytest and a knowledge of the factors that affect the results isnecessary.4. Theory4.1 The Metacenter(See Fig. 1). The transverse meta

20、-center (“M”) is based on the hull form of a vessel and is thepoint around which the vessels center of buoyancy (“B”)swings for small angles of inclination (0 to 4 unless there areabrupt changes in the shape of the hull). The location of B isfixed for any draft, trim, and heel, but it shifts appreci

21、ably asheel increases. The location of B shifts off the centerline forsmall angles of inclination (“u”), but its height above themolded keel (“K”) will stay essentially the same. The locationof M, on the other hand, is essentially fixed over a range ofheeling angles up to about 4, as the ship is inc

22、lined at constantdisplacement and trim. The height of M above K, known as“KM”, is often plotted versus draft as one of the vessels curvesof form. As a general “rule of thumb,” if the difference from thedesign trim of the vessel is less than 1 % of its length, the KMcan be taken directly from either

23、the vessels curves of form orhydrostatic tables. Because KM varies with trim, the KM mustbe computed using the trim of the ship at the time of thestability test when the difference from the design trim of thevessel is greater than 1 % of its length. Caution should beexercised when applying the “rule

24、 of thumb” to ensure thatexcessive error, as would result from a significant change in thewaterplane area during heeling, is not introduced into thestability calculations.4.2 Metacentric HeightThe vertical distance between thecenter of gravity (“G”) and M is called the metacentric height(“GM”). At s

25、mall angles of heel, GM is equal to the initialslope of the righting arm (“GZ”) curve and is calculated usingthe relationship, GZ = GM sin u. GM is a measure of vesselstability that can be calculated during an inclining experiment.As shown in Fig. 2, moving a weight (“W”) across the deck adistance (

26、“x”) will cause a shift in the overall center of gravity(GG8) of the vessel equal to (W)(x)/displ and parallel to themovement of W. The vessel will heel over to a new equilibriumheel angle where the new center of buoyancy, B8, will onceagain be directly under the new center of gravity (G8). Becauset

27、he angle of inclination during the inclining experiment issmall, the shift in G can be approximated by GM tan u and thenequated to (W)(x)/displ. Rearranging this equation slightlyresults in the following equation:GM 5W!x!displ!tan u!(1)Since GM and displ remain constant throughout the incliningexper

28、iment the ratio (W)(x)/tan u will be a constant. Bycarefully planning a series of weight movements, a plot oftangents is made at the appropriate moments. The ratio ismeasured as the slope of the best represented straight linedrawn through the plotted points as shown in Fig. 3, wherethree angle indic

29、ating devices have been used. This line doesnot necessarily pass through the origin or any other particularpoint, for no single point is more significant than any otherpoint. A linear regression analysis is often used to fit thestraight line.4.3 Calculating the Height of the Center of Gravity Abovet

30、he KeelKM is known for the draft and trim of the vesselduring the stability test. The metacentric height, GM,ascalculated above, is determined from the inclining experiment.The difference between the height KM and the distance GM isthe height of the center of gravity above the keel, KG. See Fig.4.4.

31、4 Measuring the Angle of Inclination (See Fig. 5.) Eachtime an inclining weight, W, is shifted a distance, x, the vesselwill settle to some equilibrium heel angle, u. To measure thisangle, u, accurately, pendulums or other precise instrumentsFIG. 1 Movement of the Center of Buoyancy FIG. 2 Metacentr

32、ic HeightF 1321 92 (2004)2are used on the vessel. When pendulums are used, the two sidesof the triangle defined by the pendulum are measured. (“Y”) isthe length of the pendulum wire from the pivot point to thebatten and (“Z”) is the distance the wire deflects from thereference position at the point

33、along the pendulum lengthwhere transverse deflections are measured. Tangent u is thencalculated:tan u5Z/Y (2)Plotting all of the readings for each of the pendulums duringthe inclining experiment aids in the discovery of bad readings.Since (W)(x)/tan u should be constant, the plotted line shouldbe st

34、raight. Deviations from a straight line are an indicationthat there were other moments acting on the vessel during theinclining. These other moments must be identified, the causecorrected, and the weight movements repeated until a straightline is achieved. Figs. 6-9 illustrate examples of how to det

35、ectsome of these other moments during the inclining and arecommended solution for each case. For simplicity, only theaverage of the readings is shown on the inclining plots.4.5 Free SurfaceDuring the stability test, the inclining ofthe vessel should result solely from the moving of the incliningweig

36、hts. It should not be inhibited or exaggerated by unknownmoments or the shifting of liquids on board. However, someliquids will be aboard the vessel in slack tanks so a discussionof “free surface” is appropriate.4.5.1 Standing Water on DeckDecks should be free ofwater. Water trapped on deck may shif

37、t and pocket in a fashionsimilar to liquids in a tank.4.5.2 Tankage During the IncliningIf there are liquids onboard the vessel when it is inclined, whether in the bilges or inthe tanks, it will shift to the low side when the vessel heels.This shift of liquids will exaggerate the heel of the vessel.

38、Unless the exact weight and distance of liquid shifted can beprecisely calculated, the GM from Eq 1 will be in error. FreeFIG. 3 A Typical Incline PlotFIG. 4 Relationship between GM, KM, and KGFIG. 5 Measuring the Angle of InclinationNOTERecheck all tanks and voids and pump out as necessary; redo al

39、lweight movements and recheck freeboard and draft readings.FIG. 6 Excessive Free LiquidsF 1321 92 (2004)3surface should be minimized by emptying the tanks completelyand making sure all bilges are dry or by completely filling thetanks so that no shift of liquid is possible. The latter method isnot th

40、e optimum because air pockets are difficult to removefrom between structural members of a tank, and the weight andcenter of the liquid in a full tank must be accurately determinedto adjust the light ship values accordingly. When tanks must beleft slack, it is desirable that the sides of the tanks be

41、 parallelvertical planes and the tanks be regular in shape (that is,rectangular, trapezoidal, and so forth) when viewed fromabove, so that the free surface moment of the liquid can beaccurately determined. The free surface moment of the liquidin a tank with parallel vertical sides can be readily cal

42、culatedby the equation:Mfs5 lb3/12Q (3)where:Mfs= free surface moment, ft-Ltonsl = length of tank, ft,b = breadth of tank, ft,Q = specific volume of liquid in tank (ft3/ton), and(See Annex A3 for liquid conversions or measure Qdirectly with a hydrometer.)Lton = long ton of 2240 lbs.Free surface corr

43、ection is independent of the height of thetank in the ship, location of the tank, and direction of heel.4.5.3 As the width of the tank increases, the value of freesurface moment increases by the third power. The distanceavailable for the liquid to shift is the predominant factor. Thisis why even the

44、 smallest amount of liquid in the bottom of awide tank or bilge is normally unacceptable and should beremoved before the inclining experiment. Insignificantamounts of liquids in V-shaped tanks or voids (for example, achain locker in the bow), where the potential shift is negligible,may remain if rem

45、oval of the liquid would be difficult or wouldcause extensive delays.5. Preparations for the Stability Test5.1 General Condition of the Vessel A vessel should be ascomplete as possible at the time of the stability test. Schedulethe test to minimize the disruption in the vessels delivery dateor its o

46、perational commitments. The amount and type of workleft to be completed (weights to be added) affects the accuracyNOTETake water soundings and check lines; redo Weight Movements2 and 3.FIG. 7 Vessel Touching Bottom or Restrained by Mooring LinesFIG. 8 Steady Wind From Port Side Came Up After Initial

47、 ZeroPoint Taken (Plot Acceptable)NOTERedo Weight Movements 1 and 5.FIG. 9 Gusty Wind From Port SideF 1321 92 (2004)4of the light ship characteristics, so good judgment must beused. If the weight or center of gravity of an item to be addedcannot be determined with confidence, it is best to conduct t

48、hestability test after the item is added. Temporary material, toolboxes, staging, trash, sand, debris, and so forth on board shouldbe reduced to absolute minimum during the stability test.5.2 TankageInclude the anticipated liquid loading for thetest in the planning for the test. Preferably, all tank

49、s should beempty and clean or completely full. Keep the number of slacktanks to a minimum. The viscosity of the fluid and the shape ofthe tank should be such that the free surface effect can beaccurately determined.5.2.1 Slack Tanks:5.2.1.1 The number of slack tanks should normally belimited to one pair of port and starboard tanks or one centerlinetank of the following:(a) Freshwater reserve feed tanks,(b) Fuel/diesel oil storage tanks,(c) Fuel/diesel oil day tanks,(d) Lube oil tanks,(e) Sanitary tanks, or(f) Potable water tanks.5.2.1.2 To avoid pocket